The present invention relates to a surface light emitting device that is capable of emitting light, received from LED chips that are point light sources, uniformly over a light emitting surface of a light guide plate and is used for the back light of a liquid crystal display, panel meter, indicator light and surface light emitting switch.
Recently surface light emitting device, that effects surface emission of light by receiving light from LED chips which are point light sources, has been used as light sources such as the back light of liquid crystal display. The surface light emitting device is made in such a configuration that light, which is emitted by one or more light emitting diodes and is incident on one end face of a light guide plate that has opposing principal planes, is caused to emerge from the entire surface of one of the principal planes of the light guide plate. In the surface light emitting device, it is important to extract uniform optical output from the light emerging plane, and various proposals have been made to achieve the uniformity of optical output. A surface light emitting device of the prior art as an example of the proposal is schematically shown in FIG. 19. The surface light emitting device shown in FIG. 19 comprises a light guide plate 901 that has a first principal plane and a second principal plane and is made of a light transmitting resin capable of transmitting light, a molded LED 902 that is disposed to oppose an end face of the light guide plate 901 and includes an LED element molded therein, and a reflector 904 mounted on the second principal plane of the light guide plate to reflect light so that the light emerges from the first principal plane.
In the surface light emitting device shown in FIG. 19, a notch of semi-cylindrical shape 911 is formed on the input end face of the light guide plate 901 so that light is diffused uniformly in a plane parallel to the principal plane of the light guide plate 901, thereby to cause the light entering from the LED element to be emitted from the first principal plane uniformly.
However, in the surface light emitting device of the prior art shown in FIG. 19, there has been such a problem that luminance becomes extremely high in the vicinity of the notch 911 in the first principal plane of the light guide plate.
Also in the surface light emitting device of the prior art shown in FIG. 19, although optical output is made uniform to some extent by the notch 911, it has not been sufficient.
An object of the present invention is to solve the problems described above and provide a surface light emitting device that is capable of emitting light of high luminance with higher uniformity.
The inventors of the present application conducted various experiments and researches and found that light incident on an end face of a light guide plate can be diffused more uniformly in a light guide plate by forming a light diffusing portion of a predetermined configuration on the end face of the light guide plate, thus completing the present invention.
The surface light emitting device of the present invention comprises a light guide plate having a top surface and an under surface that oppose each other, a reflector installed on the under surface of the light guide plate and an LED light source that comprises at least one LED element so that light emitted by the LED element is incident on one end face of the light guide plate, thereby to cause the light to emerge from the top surface of the light guide plate, characterized in that:
the light guide plate has such a light diffusing portion on the one end face described above, that the light emitted from the LED light source enters the light guide plate while being diffused therein, and
the light diffusing portion has the top end being formed to be space from the top surface at the one end.
In the surface light emitting device made in such a configuration as described above, since the light diffusing portion is formed to be space from the top surface of the light guide plate at the one end of the light guide plate so that an opening is not formed on the top surface of the light guide plate, light is prevented from leaking through the top surface of the light guide plate and uniform surface light emission without abnormal light emission can be achieved.
The top surface f the light guide plate refers to the surface through which the light is emitted, and the under surface refers to the surface opposite to the light emitting surface.
In the surface light emitting device of the present invention, the light diffusing portion is preferably a notch made in a shape of half pyramid or half circular cone expanding downward from an apex located below the top surface on the one end face, in order to cause the incident light to be emitted from the top surface of the light guide plate efficiently.
In the surface light emitting device of the present invention, the notch is preferably made in a triangular pyramid shape.
Also in the surface light emitting device of the present invention, the notches are preferably formed in plurality for every LED element, in order to cause the light from the LED light source to be emitted from the top surface of the light guide plate efficiently.
In the surface light emitting device of the present invention, adjacent notches among the plurality of notches formed for every LED element may be formed to be spaced from each other at the under surface of the light guide plate.
With such a configuration as described above, relatively large flat areas can be formed between the notches. This causes a part of the incident light that enters the notches to be diffused and other part of light incident on the flat surface to be transmitted forward, thus making it possible to effect uniform surface light emission over the surface in the surface light emitting device of relatively long configuration.
Moreover, in the surface light emitting device of the present invention, it is preferable that the notch has a shape of triangular pyramid having two inclined surfaces, and that the inclined surfaces that constitute at least two notches among the notches described above are formed with different angles with respect to the one end described above, so that the light from the LED light source enters the light guide plate while being more diffused.
In the surface light emitting device of the present invention, the light diffusing portion may also be formed in such a configuration as the top end and the bottom end of the light diffusing portion are located away from the top surface and the under surface at the one end face, in order to prevent leakage of the light from the LED light source through the top surface and the under surface of the light diffusing portion.
In the surface light emitting device of the present invention, inner surface of the light diffusing portion may also be a curved surface.
In the surface light emitting device of the present invention, inner surface of the light diffusing portion may also be a spherical surface.
In the surface light emitting device of the present invention, the light diffusing portion may also be made in a semi-cylindrical shape.
In the surface light emitting device of the present invention, the LED element may also be covered by a light transmitting resin.
The surface light emitting device of the present invention may also comprise an LED bar light source having a surface located to oppose the one end face described above of the light guide plate and a light emitting portion that has a concave shape formed in the surface where the LED element is mounted and is filled with a light transmitting resin. In this case, the light diffusing portion is located to oppose the light emerging surface of the light emitting portion.
In the surface light emitting device of the present invention, the LED bar light source may also have a plurality of light emitting portions. This makes it possible for the surface light emitting device to emit light of sufficient luminance from a surface of wide area.
Further in the surface light emitting device of the present invention, the light transmitting resin may include a fluorescent matter that absorbs light emitted by the LED element and emits light of a wavelength different from that of the absorbed light.
Also in the surface light emitting device of the present invention, the LED element may be a light emitting element that is capable of emitting visible light and the fluorescent matter may be a phosphor that absorbs the visible light emitted by the light emitting element and emits visible light of a wavelength longer than that of the light absorbed. With this constitution, the emitted light can be rendered a desired color tone by a proper combination of the LED element and the fluorescent substance.
Also in the surface light emitting device of the present invention, the light emitting portion may be made in such a constitution as white light generated by mixing the light emitted by the LED element and the light emitted by the fluorescent substance is output from the light emerging surface.
The surface light emitting device may also be made in such a constitution as the LED element is an LED that includes a nitride semiconductor, and the fluorescent substance is a yttrium-aluminum-garnet fluorescent substance activated with cerium, so that white light is output from the light emerging surface.